1. Technical Field of the Invention
The present invention relates to a control device for a synchronous motor used in a machine tool or the like, and particularly to a control device for a reluctance type synchronous motor.
2. Description of the Prior Art
A permanent magnet type synchronous electric motor which adopts a permanent magnet as a rotor has conventionally been used as an electric motor for positioning or shaft feeding in a machine tool or the like. To control the output torque of this motor, only the amplitude of the armature current (or torque current) needs to be controlled by a control device. In this motor, however, since the magnetomotive force of the field magnetism (or magnet) cannot be controlled, the field magnetism cannot be arbitrarily controlled using field magnetism weakening control (or rated output control) or the like. Consequently, the inductive voltage between terminals exceeds the power source voltage when the rotation speed reaches or exceeds a designed rotation speed (which will be called a base rotation speed), resulting in a problem that control of the output torque is rendered unstable.
Therefore, a reluctance type electric motor whose d-axis (or field magnetism) current and q-axis (or electric motor) current can be independently controlled is used in place of the permanent magnet type electric motor. By using this reluctance type electric motor, the d-axis current can be weakened in correspondence with the rotor speed when the rotation speed reaches or exceeds the base speed (i.e., the magnetomotive force can be reduced in the case of a permanent magnet). Thus, stable control of the output torque is achieved when the rotation speed is equal to or higher than the base speed).
FIG. 8 shows a control block diagram according to the prior art. A difference between a speed command SVC and a speed detection value SPD is calculated by a subtracter 1. Based on the difference, a PI controller 2 generates a q-axis (or electric motor) current command STC which is supplied to a current command calculator section 3 by the PI controller 2. Meanwhile, a field current calculator section 81 generates a d-axis (or field magnetism) current command SFC from a speed detection value SPD, and supplies this command to the current command calculator section 3. The current command calculator section 3 generates U-phase and V-phase current commands SIUC and SIVC from a detection value SP of the rotor position detected by a detector 6, and supplies these commands to a current control section 4. Based on the commands supplied, the current control section 4 controls the current to be supplied to an electric motor 5. Note that a differentiator 7 generates the speed detection value SPD from the detection value SP of the rotor position. In addition, the field current calculator section 81 outputs a field current command according to a function pattern shown in FIG. 3(a) (in which the function takes a constant value when the speed is equal to or lower than a base rotation speed SPDbase while the function satisfies a curve 31 when the speed is equal to or higher than the base rotation speed SPDbase.
In this conventional control device for a synchronous electric motor, field current control depends only on the rotor speed SPD. Further, field magnetism weakening control is carried out only when the rotation speed is equal to or higher than the base rotation speed SPDbase. Therefore, 100% of the field current flows even when no torque is required, e.g., while the motor is stopped. A problem therefore occurs not only in that the electric power consumption is large but also in that the electric motor itself becomes heated. If the field current is large, the torque ripple becomes large, as is apparent from the characteristic graph shown in FIG. 7. Therefore, in the case where the motor is used for a feed shaft of a machine tool, influences from the torque ripple appear as processing fringes, thus resulting in reductions in processing accuracy.
In addition, when the field magnetism is constant, the torque generated is limited, resulting in a problem that the acceleration time and deceleration time cannot be shortened. Further, when the motor current is large (e.g., where STC&gt;&gt;SFC), almost all components of a current command value as a synthesis current serve as motor current components. Therefore, when a delay of control timing occurs in a electric current control loop or the like, there occurs a problem that a current command comes to include motor current components including such field current components which may inversely rotate the rotor, resulting in a phenomenon where the rotor stops or is inversely rotated and is finally rendered uncontrollable while a command for regular rotation is generated.